In the semiconductor industry, chemical mechanical polishing (CMP) is used to selectively remove portions of a film from a semiconductor wafer by rotating the wafer against a polishing pad (or rotating the pad against the wafer, or both) with a controlled amount of pressure in the presence of a chemically reactive slurry. Overpolishing (removing too much) or underpolishing (removing too little) of a film results in scrapping or rework of the wafer, which can be very expensive. Various methods have been employed to detect when the desired endpoint for removal has been reached, and the polishing should be stopped. One such method described in copending patent application Ser. No. 08/419,206 filed Apr. 10, 1995 entitled "In-Situ Monitoring of the Change in Thickness of Films" and assigned to the present assignee, uses a resonant tank circuit type sensor which can be located near the back of the wafer during the polishing process. As the polishing process proceeds, the load from the film on the sensor circuit decreases and the sensor generates a signal whose change corresponds to the changing film thickness. In application Ser. No. 08/419,206, the sensor was excited by a swept output from an instrument such as a spectrum analyzer, function generator, or frequency oscillator. The instrument display then showed a resonant peak at the characteristic frequency associated with the sensor circuit being loaded by the film being monitored. As the film was removed, the peak sharpened, increased in amplitude, and shifted to a different frequency. The change in voltage amplitude was used in what we call "manual amplitude mode" to determine the endpoint.
There are several characteristics of manual amplitude mode that are less than desirable when endpoint detection of a chemical-mechanical polishing process is performed. First, it gives a binary response--the peak shift occurs suddenly, indicating endpoint, with no indication of gradual change. A data trace for an individual wafer would resemble a step function, with no indication of polish history, and would lack in sensitivity. Second, external noise interferes easily with the monitoring process. Third, the sensor circuit had to be tuned so that it could differentiate between a load from the wafer with and without metal, which varied according to the conductivity of the underlying wafer. Constant tuning was a result.
Thus there remains a need for an endpoint detection method and apparatus for chemical mechanical polishing that solves the above-listed problems and is suitable for use in large-scale manufacturing.